The invention relates to a superconducting device having at least one inductive current limiter unit, which contains at least one conductor track which carries a switching current in a limiting situation, at least one annular body which is inductively associated with the conductor track and is composed of high-Tc superconductor material, and a core limb which is surrounded by the annular body and is composed of soft-magnetic material. A corresponding superconducting device is disclosed in EP 0 353 449 A1. Its at least one current limiter unit may have an associated transformer, which has a primary and a secondary coil winding as well as an associated magnetic flux body composed of soft-magnetic material and having a plurality of yoke limbs.
Short circuits and electrical flashovers cannot be reliably avoided in electrical AC supply networks. In the process, the alternating current in the circuit that is affected arises very quickly, that is to say within the first half-cycle, to a multiple of its rated value, until it is interrupted by suitable protection and/or switching units. As a consequence of this, considerable thermal and mechanical stresses occur as a result of current forces in all the affected network components, such as cables, busbars, switches and transformers. Since these short-term loads increase with the square of the current, safe limiting of the short-circuit current to a lower peak value can considerably reduce the requirements for the load capacity of the network components. This results in cost advantages, for example for the construction of new networks and when upgrading existing networks, in that the installation of current limiter units makes it possible to avoid the need to replace network components by embodiments having a higher load capacity.
The use of superconducting current limiter units makes it possible, in a manner known per se, to limit the current rise after a short circuit to a value equivalent to a few multiples of the rated current. Furthermore, a limiter unit such as this is ready for operation once again a short time after the disconnection process. It thus acts as a rapid, self-healing protection device. In the process, it ensures high operational reliability since it acts passively, that is to say autonomously without previous detection of short circuits and without active tripping by a switching signal.
Superconducting current limiter units normally form a switching element which can be inserted into a circuit, in series. Appropriate current limiter units may be of the so-called resistive or inductive type. Inductive current limiter units are normally in the form of induction coils (see, for example DE 38 29 207 A1 or EP 0 440 664 B1). In this type, a shield current is inducted by a conductor track (which carries the rated current during operation) of an induction coil winding in an associated (secondary) superconducting winding, which is short-circuited. This superconducting winding may also be formed by a superconducting core or by the part of such a core in the interior of the induction coil. In the event of a short circuit, the shield current which is induced by a corresponding switching current in the superconducting winding or core becomes so great that the critical (shield) current of the superconducting material is exceeded. In consequence, the current-carrying capacity of this winding or of the core collapses as the superconducting material becomes normally conductive, as a result of which the inductance of the induction coil increases suddenly, with the impedance in the conductor track of the induction coil being raised in a corresponding manner to a value which produces the current-limiting effect. A corresponding inductive current limiter unit is disclosed in the initially cited EP-A document. This contains an induction coil through which the rated current flows and which surrounds an annular body composed of high-Tc superconductor material. This annular body has a centrally symmetrical shape, with a hollow interior, in which a core limb composed of soft-magnetic material of high permeability is arranged concentrically. This core limb may in this case also be part of a complete, intrinsically closed, magnetic circuit.
Known current limiter units such as these with metal-oxide high-Tc materials (so-called xe2x80x9cHTSxe2x80x9d materials) whose critical temperature Tc is sufficiently high that they can be maintained in the superconducting operating state by liquid nitrogen (LN2) and at most 77 K exhibit a rapid increase in the electrical resistance when their critical values are exceeded. The heating which is associated with this in the normally conductive state, and hence the indirect initiation of the current limiting process in this case take place in a sufficiently short time so that the peak value of a short-circuit current can be limited to a fraction of the unlimited current, for example to 3 to 10 times the value of the rated current. The superconducting parts should in this case make a good thermally conductive contact with a suitable coolant, which allows them to be returned to the superconducting operating state once again within a relatively short time after the critical values have been exceeded.
If a corresponding current limiter unit is now intended to be used to limit the current in the circuit of one of the coil windings in the transformer, whose at least one conductor surrounds a yoke limb of a magnetic flux body composed of soft-magnetic material, then, according to the related art, this is achieved by connecting appropriate separate components in series in the circuit. The design complexity relating to this is correspondingly high.
One potential object of the present invention is therefore to specify a superconducting device having the features mentioned initially, but whose design complexity is reduced.
In consequence, the superconducting device according to one aspect of the invention has at least one inductive current limiter unit which contains at least one conductor track which carries a switching current in a limiting situation, at least one annular body which is inductively associated with the conductor track and is composed of high-Tc superconductor material, and a core limb which is surrounded by the annular body and is composed of soft-magnetic material. The device furthermore has a transformer which has a primary and a secondary coil winding as well as an associated magnetic flux body composed of soft-magnetic material and having a plurality of yoke limbs. In this case, between the primary coil winding and the secondary coil winding, the magnetic flux body is intended to be provided with at least one further yoke limb which is provided as the core limb of the current limiter unit and, together with the at least one conductor of one of the coil windings, is intended to form the at least one conductor track of the current limiter unit.
In this context, the expression xe2x80x9cannular bodyxe2x80x9d means any structure comprising at least one element, component or conductor which contains at least high-Tc superconductor material and forms a short-circuited ring which surrounds the associated yoke limb (which acts as a magnetic bypass element) of the magnetic flux body.
The advantages associated with the refinement of the semiconductor device according to one aspect of the invention are in particular that the magnetic return path of the inductive current limiter unit is integrated, as a further yoke limb, in the magnetic flux body which needs to be provided in any case for the transformer coil windings. The amount of soft-magnetic material is thus correspondingly reduced. Furthermore, the current limiter unit no longer requires its own conductor track to produce a switching current, since the switching current is now produced by the at least one conductor of one of the coil windings themselves; that is to say the function of the high-Tc superconductor material of the annular body to initiate switching is provided solely by the conductor of this coil winding.
For example, the conductors of the transformer coil windings may particularly advantageously likewise contain high-Tc superconductor material. In this case, in particular, the coil windings and the current limiter unit may be arranged in a common cryostat vessel which contains two coolant areas in which, respectively, the winding and the current limiter unit are accommodated. This has the advantage that different temperature levels may be set in the coolant areas.
It is particularly advantageous for the coolant in the coolant area of the transformer coil windings to be at a lower temperature level than the coolant in the coolant area for the limiter unit. In this case, the coolant area for these coil windings and the coolant area for the limiter unit should preferably be at least approximately at the same pressure (including discrepancies of a maximum of xc2x110% between the pressure in the coolant area for the limiter unit and the pressure in the coolant area for the windings). This is because this makes it possible to dissipate by convection the heat losses produced in the windings by virtue of the alternating current losses in the superconductor without this leading to undesirable gas formation resulting from corresponding vaporization. This makes it possible to avoid the electrical withstand voltage, which is a particular requirement for operation at high voltage, being reduced by gas bubbles.
The superconducting device may also advantageously have a current limiter unit which has a plurality of yoke limbs, which are surrounded by superconducting annular bodies, of a magnetic flux body. This provides a correspondingly high level of design freedom with regard to the cross section of the soft-magnetic material.